Electric machines, such as DC brushless motors, and the like, may be used in an increasing variety of industries and applications where a high motor output, superior efficiency of operation, and low manufacturing cost often play a critical role in the success and environmental impact of the product, e.g., robotics, industrial automation, electric vehicles, HVAC systems, appliances, power tools, medical devices, and military and space exploration applications. These electric machines typically operate at frequencies of several hundred Hz with relatively high iron losses in their stator winding cores and often suffer from design limitations associated with the construction of stator winding cores from laminated electrical steel.
A typical brushless DC motor includes a rotor, with a set of permanent magnets with alternating polarity, and a stator. The stator typically comprises a set of windings and a stator core. The stator core is a key component of the magnetic circuit of the motor as it provides a magnetic path through the windings of the motor stator.
In order to achieve high efficiency of operation, the stator core needs to provide a good magnetic path, i.e., high permeability, low coercivity and high saturation induction, while minimizing losses associated with eddy currents induced in the stator core due to rapid changes of the magnetic field as the motor rotates. This may be achieved by constructing the stator core by stacking a number of individually laminated thin sheet-metal elements to build the stator core of the desired thickness. Each of the elements may be stamped or cut from sheet metal and coated with insulating layer that prevents electric conduction between neighboring elements. The elements are typically oriented in such a manner that magnetic flux is channeled along the elements without crossing the insulation layers which may act as air gaps and reduce the efficiency of the motor. At the same time, the insulation layers prevent electric currents perpendicular to the direction of the magnetic flux to effectively reduce losses associated with eddy currents induced in the stator core.
The fabrication of a conventional laminated stator core is complicated, wasteful, and labor intensive because the individual elements need to be cut, coated with an insulating layer and then assembled together. Furthermore, because the magnetic flux needs to remain aligned with the laminations of the iron core, the geometry of the motor may be considerably constrained. This typically results in motor designs with sub-optimal stator core properties, restricted magnetic circuit configurations, and limited cogging reduction measures critical for numerous vibration-sensitive applications, such as in substrate-handling and medical robotics, and the like. It may also be difficult to incorporate cooling into the laminated stator core to allow for increased current density in the windings and improve the torque output of the motor. This may result in motor designs with sub-optimal properties.
Soft magnetic composites (SMC) include powder particles with an insulation layer on the surface. See, e.g., Jansson, P., Advances in Soft Magnetic Composites Based on Iron Powder, Soft Magnetic Materials, '98, Paper No. 7, Barcelona, Spain, April 1998, and Uozumi, G. et al., Properties of Soft Magnetic Composite With Evaporated MgO Insulation Coating for Low Iron Loss, Materials Science Forum, Vols. 534-536, pp. 1361-1364, 2007, both incorporated by reference herein. In theory, SMC materials may offer advantages for construction of motor stator cores when compared with steel laminations due to their isotropic nature and suitability for fabrication of complex components by a net-shape powder metallurgy production route.
Electric motors built with powder metal stators designed to take full advantage of the properties of the SMC material have recently been described by several authors. See, e.g., Jack, A. G., Mecrow, B. C., and Maddison, C. P., Combined Radial and Axial Permanent Magnet Motors Using Soft Magnetic Composites, Ninth International Conference on Electrical Machines and Drives, Conference Publication No. 468, 1999, Jack. A. G. et al., Permanent-Magnet Machines with Powdered Iron Cores and Prepressed Windings, IEEE Transactions on Industry Applications, Vol. 36, No. 4, pp. 1077-1084, July/August 2000, Hur, J. et al., Development of High-Efficiency 42V Cooling Fan Motor for Hybrid Electric Vehicle Applications, IEEE Vehicle Power an Propulsion Conference, Windsor, U.K., September 2006, and Cvetkovski, G., and Petkovska. L., Performance Improvement of PM Synchronous Motor by Using Soft Magnetic Composite Material, IEEE Transactions on Magnetics, Vol. 44, No. 11, pp. 3812-3815, November 2008, all incorporated by reference herein, reporting significant performance advantages. While these motor prototyping efforts demonstrated the potential of isotropic materials, the complexity and cost of the production of a high performance SMC material remains a major limiting factor for a broader deployment of the SMC technology.
For example, in order to produce a high-density SMC material based on iron powder with MgO insulation coating, the following steps may be required: 1) iron powder is produced, typically using a water atomization process, 2) an oxide layer is formed on the surface of the iron particles, 3) Mg powder is added, 4) the mixture is heated to 650° C. in vacuum, 5) the resulting Mg evaporated powder with silicon resin and glass binder is compacted at 600 to 1,200 MPa to form a component; vibration may be applied as part of the compaction process, and 6) the component is annealed to relieve stress at 600° C. See, e.g., Uozumi. G. et al., Properties of Soft Magnetic Composite with Evaporated MgO Insulation Coating for Low Iron Loss, Materials Science Forum, Vols. 534-536, pp. 1361-1364, 2007, incorporated by reference herein.